Fix branches autocomplete paths in the merge request list app

What does this MR do and why?

This MR fixes #557987 (closed).

It does so by providing autocomplete paths from backend, which respects the relative_url_root setting.

CleanShot_2025-09-01_at_18.11.27

How to set up and validate locally

1. Run gdk config set relative_url_root '/test' && gdk reconfigure && gdk restart
2. Open any project with multiple branches and at least 1 merge request
3. Open merge requests list
4. Select the filter bar and select 'Target branch' filter
5. Select 'equals'
6. Observe that branches autocomplete works
7. Run gdk config set relative_url_root '' && gdk reconfigure && gdk restart to restore to the previous state